This invention relates to methods of producing polyethylene-based coatings for electrical conductor cable, and in particular to new and improved ways of preparing polyethylene-based compositions such as are used to coat cable, as well as to intermediate compositions having utility therefor.
For purposes of illustration, detailed reference will be made herein to the provision of electrically insulating coatings for cable, as representing one important field of use of the type of polyethylene-based compositions with which the invention is concerned. It will be understood that the term "cable" embraces both single- and multi-strand metal electrical conductors, regardless of gauge or cross-sectional shape.
A typical conventional polyethylene-based cable-coating composition comprises a major proportion of polyethylene and minor proportions of other ingredients (herein collectively termed "additive material") such as fillers, anti-oxidants, and cross-linking agents. The polyethylene content may be a homopolymer, or a copolymer with a minor amount (e.g. about 11% by weight) of ethylene vinyl acetate; the term "polyethylene" is used herein generically to designate both homopolymers of polyethylene, and copolymers of a major proportion of polyethylene and a minor proportion of ethylene vinyl acetate. Commonly, the principal additive is the filler; nonconductive carbon black is frequently preferred as a filler, because it affords a measure of protection against ultraviolet radiation, but other solid particulate fillers such as clays are also sometimes used.
In present-day commercial practice, a coating composition as described above is ordinarily prepared by mixing all the ingredients together (at a temperature at which the polyethylene is fluid) in heavy-duty mixing equipment of the type known as a Banbury mixer, which is capable of providing the high mechanical shear mixing action conventionally needed to blend particulate fillers and/or other additive material in polyethylene, i.e. for attainment of the high degree of homogeneity required for cable coating compositions. The composition is screened after mixing to remove any unblended material. If a cross-linking agent is used, it is added to the composition in a second mixing step (also conventionally performed in a Banbury mixer, although high shear is not required) after the other ingredients have been blended, because the high temperature developed in initially mixing ingredients such as particulate fillers with polyethylene would result in premature curing if the cross-linking agent were then present. Finally, the fully-mixed composition is pelletized and delivered to extruding apparatus having a screw-type pump which forces the composition at elevated temperature through a die in surrounding relation to an advancing cable so as to coat the cable surface; in cases where a cross-linking agent is used, the coating is subsequently cured by passing the coated cable through a curing chamber such as a steam tube.
The step of fully premixing the entire coating composition in a Banbury mixer before the composition is supplied to an extruder contributes significantly, and undesirably, to the overall expense of producing polyethylene-based cable coatings, owing to the high capital and operating costs of such mixing equipment. The requirement that the coating composition be very homogeneously blended, and the difficulty of mixing additive material with polyethylene in conventional practice, however, have heretofore necessitated use of that mixing step; for example, the pumping screw of an extruder (which is designed to pump rather than to mix) is not capable of adequately blending a conventional polyethylene-based cable-coating composition. Thus, in current practice, a coated-cable producer must either invest in or rent large-capacity Banbury mixers, or else purchase his requirement of premixed coating composition from a mixer-supplier; in the latter case, all the polyethylene used in the composition must be shipped first to the mixer-supplier from a polyethylene producer, and then (as fully mixed coating composition) shipped to the coated-cable producer from the mixer-supplier, with attendant high freight costs. For reasons of economy as well as operating convenience, it would be desirable to reduce the amount of material required to be treated in a Banbury mixer for production of a given quantity of cable coating, if that could be done without sacrifice of the requisite homogeneity of blending.